The aeronautical industry is often faced with the need to reduce the level of vibrations generated by the rotating masses of an aircraft and transmitted to structural parts thereof.
In particular, in the case of propeller driven aircraft, the propeller blades generate pressure waves in the air surrounding the aircraft, propagating according to known laws and which give rise to periodic loads of significant magnitude on the fuselage and which generate the characteristic low frequency noise of such type of aircraft.
A frequency analysis of the amplitude of the pressure waves shows that the fundamental frequency has the same frequency as the angular velocity of the rotating propeller multiplied by the number of propeller blades and is generally of the order of tens of Hertz; also the first harmonic has a considerable amplitude and has a frequency twice that of the fundamental.
To cut back the noise mentioned above, at present use is made of dynamic dampers, having a resonating mass, tuned to a specific frequency and anchored to the structure whose vibration to be damped. Therefore to damp more than one frequency, entrusted to two sets of different dampers, tuned to the respective frequencies, are provided.
This causes the obvious inconvenience of fitting double dampers, each manufactured to high precision standards and each individually tuned to the required frequency to compensate for the inevitable spread of the resonating frequencies due to manufacturing tolerances and installation inaccuracies.